Microwave monolithic integrated circuits (MMIC) made of gallium arsenide (GaAs) are well known in the art and typically have a configuration similar to that which is shown in FIG. 1. More specifically, a GaAs MMIC 10 generally comprises a GaAs substrate 11 having an active region 12 and first and second passive regions 13, 14. The active region 12 may, for example, comprise at least one MESFET having three electrodes: a drain 15a, a gate 15b and a source 15c. The drain 15a and the source 15c may be respectively connected to the microwave band matching circuits comprising the passive regions 13, 14 by means of metallized bridges (not shown in FIG. 1). Constituted as such, the GaAs MMIC 10 may be used as a high frequency amplifier.
In a GaAs MMIC used as a high output amplifier, the operational temperature of the active region 12 may exceed 100.degree. C. Therefore, it is important that the MMIC be provided with means for radiating the heat, thereby enhancing its operational characteristics and reliability. In the prior art GaAs MMIC shown in FIG. 1, heat generated in the active region 12 radiates downwardly through the GaAs substrate 11. Since GaAs has a low thermal conductivity, however, the heat radiation characteristics of this prior art GaAs MMIC are inadequate.
Another prior art GaAs MMIC, in which a relatively good heat radiation effect is obtained, is shown in FIG. 2C. GaAs MMICs of such construction are disclosed in "A Packaged 20-GHz 1-W GaAs MESFET with a Novel Via-Hole Plated Heat Sink Structure" and "A K-Band GaAs FET Amplifier with 8.2-W Output Power", which appeared in IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-32, No. 3, March 1984 at pages 309-316 and 317-324, respectively.
A method of producing this type of GaAs MMIC is illustrated in FIG. 2. First, as shown in FIG. 2A, a coating of wax 16 is provided over the entire surface of the MMIC to protect the active region 12 and the passive regions 13, 14, and a glass plate 17 is provided thereon. The GaAs substrate 11 is then reduced to a thickness of approximately 50 microns by grinding, for example. Thereafter, titanium and gold are successively plated on the GaAs substrate 11, thereby producing a titanium cladding layer 18 and a gold cladding layer 19. Next, as shown in FIG. 2B, a plated heat sink (PHS) 20 having a thickness of approximately 50 microns is electrolytically plated on the gold cladding layer 19. Preferably, the plated heat sink should be comprised of a material having high thermal conductivity, such as gold. To complete the production of the GaAs MMIC, the glass plate 17 and the wax 16 are removed (see FIG. 2C).
In a GaAs MMIC having a structure such as described above and shown in FIG. 2C, heat generated in the active region 12 is more efficiently radiated through the PHS 20. However, due to the substantially decreased thickness of the GaAs substrate underlying the passive regions, the pattern sizes of the microwave transmission lines in those regions must be narrowed to achieve proper impedance matching. Consequently, the transmission losses of the lines are increased.